An electric belt retractor having a belt spool which is coupled to an electric motor and can be driven in the direction of belt retraction and/or belt withdrawal, is part of a vehicle occupant restraint system with which various precautions can be taken on detection of a critical vehicle situation in order to protect a vehicle occupant from negative consequences in the best possible way. The steps taken may comprise, for example, changing the inclination of a back rest of a vehicle seat, closing vehicle windows, pre-tensioning the safety belt and further measures. All these measures are reversible and repeatable; as soon as the vehicle is in a stable state again, the vehicle is returned into its initial state.
As regards the belt retractor, this means that the safety belt, which was wound up by the electric motor on detection of a critical vehicle situation in order to eliminate the so-called belt slack in the safety belt as largely as possible, is released again as soon as the vehicle state which was detected as being critical is no longer present.
During or after the tensioning process, a vehicle-sensitive blocking of the belt spool can occur, for example with a braking of the vehicle. In a belt retractor with a vehicle-sensitive blocking catch, this means a complete steering in of the blocking catch into blocking teeth such that a rotation of the belt spool is blocked in the belt band withdrawal direction. The blocking teeth are usually undercut and therefore exert a force drawing the blocking catch into the blocking teeth on stressing, so as to be able to reliably transfer the high blocking forces which prevail in an accident between the blocking teeth and the blocking catch.
In order to make it possible for the vehicle occupant to subsequently extract belt band again when the vehicle state which was detected as being critical is no longer present, the blocking catch which was steered in. must be released from the blocking teeth (unlocking). For this, a further brief operating of the electric motor is necessary in the belt band retraction direction, in order to turn the belt spool with the blocking teeth (or with the blocking catch) about a particular angle such that the blocking catch can fall out from the blocking teeth. The unlocking operation of the electric motor is terminated by means of a sensor on reaching a predetermined angle.
The rotation of the belt spool causes an additional belt band retraction, which is felt to be uncomfortable by the vehicle occupant. However, the unlocking operation of the electric motor takes place independently of whether the blocking catch was in fact drawn into the blocking teeth or not, because the state of the blocking catch is not monitored. This means that the vehicle occupant in any case comes to feel an additional rise in the belt band force after a tensioning of the belt, although this is unnecessary in many cases.
It is an object of the invention to provide an electric belt retractor with reduced stress and improved comfort for the vehicle occupant.